Light emitting diodes (LEDs) are being used as light sources in an increasing variety of applications extending from communications and instrumentation to household, automotive and other visual displays. LED arrays comprise a plurality of LEDs arranged on a common substrate. One problem with LED arrays is the significant heat generated by dense concentrations of LEDs. Solutions to the thermal problems associated with LED arrays are the subject of a related application entitled, “Light Emitting Diodes Packaged For High Temperature Operation” U.S. patent application Ser. No. 10/638,579, filed Aug. 11, 2003. The Ser. No. 10/630,579 application is incorporated herein by reference.
Another problem in LED arrays concerns illumination efficiency. Illumination efficiency is a measure of the percentage of generated light that actually leaves an LED package and that can serve as useable light in the intended application. FIGS. 1 and 2 show a typical LED array 10. LED dies (semiconductor chips) 12 generate light. LED dies 12 are typically box-like in structure with 6 sides. Since they are almost always mounted on one of the light surfaces, the other 5 surfaces are capable of emitting light generated by the device. Some of the light is absorbed by nearby walls 22 of array package 11, some is reflected back to the emitting die, and some is absorbed directly by nearby LED die 12 in the array. The remainder of the light exits the package.
There is a relationship between illumination efficiency and the thermal problems of LED arrays. Self-heating by absorption contributes to thermal problems. Thus, there is a need for an LED packaging arrangement that can increase the illumination efficiency of LED array devices and reduce the thermal problems produced by absorption.